Fluorescence microscopy images the distribution of fluorescent molecules in a sample. Structured illumination microscopy (SIM) achieves a two-fold increase in spatial resolution over normal wide-field microscopy by modulating the cell sample fluorophore distribution with a spatially periodic optical signal. This creates side bands which allow access to down-modulated high spatial frequencies that are normally blocked by the finite transmission function of optical microscopes.
The periodic patterns are achieved by illuminating the sample with two or more different laser beams that interfere at the sample. Generally, these beams are chosen to be collimated. Collimated beams emerging from the microscope objective towards the sample correspond to beams that are focused on the back focal plane of the objective. Hence, the illumination light source typically requires a finite set of laser beams to be focused on the back focal plane of the objective and that the interfering beams be moved across the sample.